1. Field of the Invention
The present invention relates to an image display apparatus, and in particular, to an image display apparatus using an image display medium which can be repeatedly rewritten upon by driving colored particles using an electric field.
2. Description of the Related Art
Conventionally, there is an image display medium using colored particles, so-called electronic paper, which can be repeatedly rewritten upon, and which has memory-like characteristics. An image display apparatus, using such an image display medium having memory-like characteristics, is effective for power-saving because it suffices that electric power is supplied to the image display medium only at the time of switching displays.
A method in which a color filter is disposed on a top-plane substrate has been commonly used in order to display a color image on such an image display apparatus using colored particles. However, there has been a problem that whiteness and resolution of a black-and-white image deteriorate in this method. Therefore, the present inventors proposed a method in which color display is carried out without quality deterioration of a black-and-white image, by coloring a surface of a back-plane substrate, and then carrying out color display using the back-plane substrate in addition to color display using the particles. This method is described in Japanese Patent Application Laid-Open (JP-A) No. 2002-169191, and the like.
In a conventional method, a color of the particles is displayed by making the particles adhere to the top-plane substrate. In addition to using such a conventional method, in the above-described method, the particles are moved and concentrated in a direction horizontal to a surface of the substrate, and particles in a desired region are withdrawn. Thus, a color of the surface of the back-plane substrate is displayed via the transparent top-plane substrate. For example, it is possible to display in three colors, by using colors of two types of insulating particles and the color of the back-plane substrate.
Such an image display apparatus 200 is shown in FIG. 21. As shown in FIG. 21, the image display apparatus 200 includes an image display medium 202 and a voltage applying section 204. Only one cell (pixel) is shown in FIG. 21.
In the image display medium 202, a spacer 210 is provided between a transparent top-plane substrate 206 forming an image display surface and a back-plane substrate 208. In the cell formed by the spacer 210, insulative black particles 212 which are positively charged and insulative white particles 214 which are negatively charged are enclosed.
The top-plane substrate 206 has a transparent display-side electrode 218 and a transparent insulating layer 220, which are formed on a transparent substrate 216.
The back-plane substrate 208 has an electrode layer 228 including a colored inner electrode 224 and an outer electrode 226, a colored layer 230, and a transparent insulating layer 232, which are formed on a substrate 222. The display-side electrode 218: is grounded, and the inner electrode 224 and the outer electrode 226 are connected to the voltage applying section 204.
In order to carry out white display in such an image display apparatus 200, the voltage applying section 204 applies a negative DC voltage (for example, −200V) to each of the inner electrode 224 and outer electrode 226. Due to an effect of an electric field generated between the substrates, the negatively electrified white particles 214 at the back-plane substrate 208 move to the top-plane substrate 206, and the positively electrified black particles 212 at the top-plane substrate 206 move to the back-plane substrate 208, as shown in FIG. 21. On the other hand, in order to carry out black display, the voltage applying section 204 applies a positive DC voltage (for example, +200V) to each of the inner electrode 224 and outer electrode 226.
Further, in order to make the black particles 212 and the white particles 214 withdraw to a periphery of the cell and display a color of the colored layer 230, the voltage applying section 204 applies a predetermined alternating voltage (for example, ±200V, frequency 100 Hz) to the inner electrode 224, and grounds the outer electrode 226. As a result, the black particles 212 and the white particles 214 move in a direction parallel to the substrate surface, and are concentrated on the grounded outer electrode 226, i.e., in the space between the substrates where no electric field is generated. Accordingly, particles are hardly present on the inner electrode 224, and the color of the colored layer 230 is visible from the top-plane substrate 206.
Further, when the predetermined alternating voltage is applied to the inner electrode 224 and the outer electrode 226, which are in the above-described state, the particles concentrated on the outer electrode 226 are dispersed, and the distribution of the particles can be returned to a uniform state.
However, in the above-described art, the plurality of insulative black particles 212 and the plurality of insulative white particles 214 are positively or negatively charged, respectively. Therefore, an alternating voltage must be used as a voltage applied for displaying the color of the colored layer 230 by making the particles withdraw to the periphery of the cell, and as a voltage applied for triboelectrically charging the particles by stirring the same.
In order to rapidly move a plurality of particles whose electrification polarities are different from each other to display the color of the colored layer 230, it is necessary to form a strong alternating electric field between the substrates. Further, in order to display the color of the particles, it is necessary to form a DC electric field between the substrates to move the particles to the top-plane substrate 206.
Therefore, as shown in FIG. 21, a DC power source 234 generating a DC voltage and an alternating power source 236 generating an alternating voltage are necessary for the voltage applying section 204. As shown in FIG. 22, in order to generate an alternating voltage, an inverter 238, a resistor 240, a capacitor 242, and the like are necessary for a power circuit, thus making the circuit structure complex and raising costs. Further, when the strengths of a DC electric field and an alternating electric field are different, a structure in which strength of an electric field is controlled is required. Because it is necessary to provide the power circuit with devices such as a load, a bleeder or the like, for an electric current to flow, original power-saving feature is diminished.
Moreover, an amount of charge that is applied to the particles is determined by an amount of friction or materials contained in the particles, and there are many cases in which the charged amount is not uniform, which can be an unstable factor from the standpoint of image quality. Therefore, in the prior art, control or the like for triboelectrically charging the particles by stirring the same are necessary, whereby extra electric power is consumed due to the control which does not relate to an image display.
The invention has been made in consideration of the above-described problems. An object of the invention is to provide an image display apparatus in which a color of a back-plane substrate can be displayed by moving particles to the periphery, as well as in which power-saving is achieved and control can be simplified.